Add support for handling runtime service requests

This patch uses the reworked exception handling support to handle
runtime service requests through SMCs following the SMC calling
convention. This is a giant commit since all the changes are
inter-related. It does the following:

1. Replace the old exception handling mechanism with the new one
2. Enforce that SP_EL0 is used C runtime stacks.
3. Ensures that the cold and warm boot paths use the 'cpu_context'
   structure to program an ERET into the next lower EL.
4. Ensures that SP_EL3 always points to the next 'cpu_context'
   structure prior to an ERET into the next lower EL
5. Introduces a PSCI SMC handler which completes the use of PSCI as a
   runtime service

Change-Id: I661797f834c0803d2c674d20f504df1b04c2b852
Co-authored-by: Achin Gupta <achin.gupta@arm.com>
diff --git a/common/psci/psci_private.h b/common/psci/psci_private.h
index de9c291..3d7ae74 100644
--- a/common/psci/psci_private.h
+++ b/common/psci/psci_private.h
@@ -151,6 +151,16 @@
 			       int,
 			       int);
 extern unsigned int psci_afflvl_suspend_finish(unsigned long, int, int);
+
+/* Private exported functions from psci_main.c */
+extern uint64_t psci_smc_handler(uint32_t smc_fid,
+				 uint64_t x1,
+				 uint64_t x2,
+				 uint64_t x3,
+				 uint64_t x4,
+				 void *cookie,
+				 void *handle,
+				 uint64_t flags);
 #endif /*__ASSEMBLY__*/
 
 #endif /* __PSCI_PRIVATE_H__ */